Causal Bulk Viscous Dissipative Isotropic Cosmologies with Variable Gravitational and Cosmological Constants

TL;DR

This paper investigates the evolution of a flat FRW universe with causal bulk viscous fluid, variable gravitational and cosmological constants, deriving exact solutions that describe different expansion scenarios including inflationary and non-inflationary models.

Contribution

It introduces new exact solutions for cosmologies with variable G and Λ, incorporating causal bulk viscosity and energy conservation assumptions.

Findings

Universe can end in inflationary or non-inflationary expansion.

Cosmological constant decreases over time, G increases initially then stabilizes.

Models show variable G and Λ consistent with observational constraints.

Abstract

We consider the evolution of a flat Friedmann-Robertson-Walker Universe, filled with a causal bulk viscous cosmological fluid, in the presence of variable gravitational and cosmological constants. The basic equation for the Hubble parameter, generalizing the evolution equation in the case of constant gravitational coupling and cosmological term, is derived, under the supplementary assumption that the total energy of the Universe is conserved. By assuming that the cosmological constant is proportional to the square of the Hubble parameter and a power law dependence of the bulk viscosity coefficient, temperature and relaxation time on the energy density of the cosmological fluid, two classes of exact solutions of the field equations are obtained. In the first class of solutions the Universe ends in an inflationary era, while in the second class of solutions the expansion of the Universe is non-inflationary for all times. In both models the cosmological "constant" is a decreasing function of time, while the gravitational "constant" increases in the early period of evolution of the Universe, tending in the large time limit to a constant value.